It is known that resilient bulking fibers are useful in the preparation of bulkier and more absorbent fiber structures. Such fiber structures may be useful in the manufacture of products such as hand sheets, towels, tissues, filters, paper board, diapers, sanitary napkins, hospital dressings and the like. One method of obtaining resilient bulking fibers is by crosslinking cellulose fibers by a treatment with a crosslinking agent. Such high bulk fibers typically have primarily intra-fiber crosslink bonds within a fiber. That is, the crosslink bonds are primarily between cellulose molecules within a single fiber, rather than between cellulose molecules of separate fibers. One of the characteristics of such fibers is their bulk which can be measured by the Fiber Absorption Quality Analyzer (Weyerhaeuser Co., Federal Way, Washington) (FAQ). This test is described in U.S. Pat. No. 6,184,271 and is incorporated herein by reference. In certain applications, it is desired to maximize the bulk of the fibers.
Through their prior work, the present inventors observed that FAQ values can be impacted by the amount of crosslinking agent and the solution strength added to a mat of cellulose fibers to be crosslinked. The amount of crosslinking agent and the solution strength added to a mat of cellulose fibers to be crosslinked cannot be so large that the consistency of the fiber sheet exceeds certain limits. Given such limits, in order to add as much crosslinking agent to the fiber sheet as possible, the crosslinking agent should be distributed completely and uniformly throughout the sheet.
Sheets of cellulose fibers to which crosslinking agent can be applied are provided by a wet laid sheet manufacturing line such as a pulp sheet manufacturing line. In such a process, a pulp slurry is delivered from a head box through a slice onto a Fourdrinier wire. The pulp slurry includes cellulose fibers such as wood pulp fibers and may also include synthetic or other noncellulose fibers. Water is drawn from the pulp slurry deposited on the Fourdrinier wire by a vacuum system, leaving a deposited pulp sheet which is carried through press sections to further remove the water from the fiber sheet. From the press sections, the pulp sheet enters a drying section further remove latent water. The pulp sheet follows a serpentine path through the dryer sections and exits as a dried sheet of cellulose fibers. The dried pulp sheet generally has a maximum moisture content which is no more than about 15% by weight of the fibers. If more than 15% by weight moisture is present in the sheet of cellulose fibers, the fibers tend to be too damp and are thus more susceptible to degradation by mold or the like during storage. The dried sheet can be taken up on a roll or delivered to other unit operations for further processing, such as crosslinking.
Referring to FIG. 1, high bulk fibers can be produced by a method that can be summarized as follows. Sheets of uncrosslinked cellulose fibers from rolls 124 are treated with a chemical crosslinking agent at location 118. The treated sheets are then delivered to a hammermill 208 where the individual cellulose fibers are separated into substantially unbroken individualized cellulose fibers treated with the crosslinking agent. The substantially individualized fibers exit hammermill 208 at exit 230 and are delivered via conduits 232 and 236 to cyclone 226. Cyclone 226 separates the fibers from the air carrying the fibers. The fibers drop from outlet 242 of cyclone 226 and are fed to dryer 246 for flash evaporation of residual moisture. Curing of the crosslinking material occurs subsequent to the drying of the fibers. The resulting crosslinked cellulose fibers can then be collected and packaged. U.S. Pat. No. 5,641,561 is referenced for additional detail regarding an exemplary crosslinking process.
In one process for applying a crosslinking agent to a sheet of cellulose fibers, a sheet of fibers in a vertical orientation is passed through a reservoir of the crosslinking material. From the reservoir, the sheet including the cellulose fibers and crosslinking agent is delivered to a press which applies a load to the wet sheet. The press serves not only to transport the sheet but also to encourage the crosslinking agent to distribute itself throughout the mat of cellulose fibers. While effective at introducing some crosslinking agent into the sheet of cellulose fibers, this process has been observed to result in incomplete distribution and/or uneven distribution of crosslinking agent in the sheet of cellulose fibers. Accordingly, the amount of crosslinking applied to the sheet is less than optimal. In addition, delamination of the mat of fibers was observed when the mat exited the press due to air being entrained in the treated sheet from the wall of crosslinking agent on both sides of the sheet. The amount of crosslinking agent impregnated into the sheet is also impacted by the speed of the sheet as it moves through the crosslinking agent reservoir.
In view of the recognition that the bulk, as measured by the FAQ test, is optimized when the crosslinking agent is fully and evenly distributed within a sheet of cellulose fibers from which the high bulk fibers are produced, a need exists for methods and systems to achieve more complete and uniform distribution levels of crosslinking agents than those achieved by prior impregnation techniques.
Furthermore, in view of the high cost associated with the installation of a physical facility to manufacture high bulk fibers, such method and systems would desirably be compatible with existing high bulk fiber production methods and hardware.